When operating a functional circuit, it requires excessive setting time for the functional circuit to stabilize, and this has been a problem to be solved in the field. FIG. 1 illustrates an output voltage-time curve of a functional circuit according to prior art. In FIG. 1, the functional circuit may be a phase locked loop circuit as an example. The horizontal axis is time, and the vertical axis refers to an output voltage of a phase locked loop circuit. As shown in FIG. 1, when the functional circuit is initialized, the output voltage is unstable during a time interval T11. The output voltage is unstable because the negative feedback mechanism of the functional circuit has not yet stabilized.
As shown in FIG. 1, after the time interval T11 has elapsed, it still requires a time interval T12 for the output voltage to stabilize. Thus the setting time to stabilize the output voltage is at least a sum of the time intervals T11 and T12. This makes the functional circuit highly inefficient. Hence, a solution is required for solving a problem of that the setting time is excessive.